You have to run beamer twice to get the image centered in the right place. But if you have any other aux-file tricks (e.g., table of contents) you need to do that anyway.

If you're not already using tikz, you can save your image as a pdf and then use \includepdf (part of the pdfpages package). This will get you into trouble if you want to print your slides as an article or handout, though.

An more light-weight approach (no extra packages, no necessity to compile twice) is to set the page-filling image as background template via \usebackgroundtemplate and then insert an empty [plain] frame.

To restrict the effect of the background changing to a single slide only, we do this inside a TeX group:

Is the aspect ratio of your image the same as that of your slides? If not then you'll always get either borders or clipping of the image if you include it without changing its aspect ratio.

You can put \hspace{-1.2cm} before your \includegraphics command to place the image 1.2 cm to the left of where it would be by default. Playing with the distance should let you place it right on the edge of the slide.

I am not exactly sure, but I think I read that using \setbeamertemplate is better than `usebackgroundtemplate`, due to a beamer package update. TeXstudio, for example, highlights it as a warning.
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SantiagoJun 2 at 23:25